The present invention relates generally to instrumentation receivers and more particularly to an instrumentation receiver for measuring the signal quality of digitally modulated radio frequency signals, such as generated in digitally modulated television broadcasting.
The Federal Communications Commission has adopted the Digital television Standard developed by the Advanced Television Systems Committee (ATSC). The Digital Television Standard is designed to transmit high quality video and audio and ancillary data over a 6 MHz channel. The Standard describes the channel coding and modulation RF/transmission subsystems for terrestrial and cable applications. The modulation subsystem uses a digital data stream to modulate the transmitted signal and may be implemented in two modes: a terrestrial broadcast mode (8-VSB) delivering about 19 Mbps, and a higher data rate mode (16-VSB) delivering about 38 Mbps for cable television systems where higher signal to noise ratio is ensured.
The modulation technique implemented in the Digital Television Standard employs vestigial sideband modulation that was developed by Zenith, Electronics Corp. The overall system response of the combined transmitter and receiver utilizes a raised cosine filter to eliminate inter-symbol interference. The system response is implemented with identical root raised cosine filters in the transmitter and in the receiver. The incoming digital data stream is randomized, forward error corrected (FEC) and interleaved. The randomized, FEC coded and interleaved data is trellis encoded as an 8-level (3-bit) one dimensional constellation. The outputs of the trellis coder are referred to as symbols that are one of eight discrete odd integers levels from -7 to +7 set by the encoder. To aid synchronization in low signal to noise and/or high multipath situations, segment and field syncs are added to the 10.76 Msymbols/sec signal as well as a small pilot tone at the carrier frequency generated by offsetting the real or I-channel of the composite signal containing the data and the sync pulses by 1.25 units. At the transmitter, the composite signal passes through a root raised cosine filter and modulates an intermediate frequency carrier signal which is up-converted to an RF frequency for transmission at the desired channel frequency. The offset causes the pilot tone to be in-phase with the I-channel carrier frequency. Alternately, the composite signal may directly modulate the RF carrier.
Referring to FIG. 1, there is shown a representative block diagram of a VSB receiver for extracting the digital television signal data from the digitally modulated RF signal as described in the "Guide to the Use of the ATSC Digital Television Standard" published by the ATSC. The receiver 10 receives the UHF or VHF signal through a band-pass and broadband tracking filter 12. A wideband amplifier 14 increases the signal and couples it to a first mixer 16. The mixer is driven by a 1st local oscillator 18 that tunes over a range from 978 to 1723 MHz. The 1st local oscillator 18 is synthesized by a phase locked-loop and controlled by a microprocessor (not shown). The output of the mixer 16 is an up-converted intermediate frequency (IF) signal at 920 MHz. The IF signal is coupled to an LC filter 20 in tandem with a band-pass ceramic resonator filter 22 centered at 921 MHz. An IF amplifier 24 is placed between the two filters. The IF signal is coupled to a second mixer 26 that is driven by a 2nd local oscillator 28. The 2nd local oscillator 28 is an 876 MHZ voltage controlled SAW oscillator controlled by a frequency and phase-locked loop (FPLL) synchronous detector 30. The output of the second mixer 26 is centered at 45 MHz. This IF signal drives a constant gain MHZ amplifier 32. The output of the amplifier 32 is coupled to an IF SAW filter 34. The IF SAW filter 34 implements an approximation of the matched root raised cosine filter at the receiver. The output of the SAW filter 34 is coupled to the FPLL synchronous detection circuitry 30 via an AGC controlled amplifier 36.
Carrier recovery is performed on the pilot signal by the FPLL synchronous detector circuit 30. The operation of this circuit is described in U.S. Pat. No. 4,091,410, assigned to Zenith, Electronic Corp. The configuration provides a Phase Locked Loop (PLL) function with a very wide pull-in range which insures rapid carrier acquisition. The I-channel composite baseband data signal from the FPLL synchronous detector 30 is coupled through a low pass filter 54 to an analog-to-digital converter (A/D) 56 that is clocked by a properly phased 10.76 MHz symbol clock 58. The digital data from the A/D converter 56 is coupled to a data segment sync detector 60 having a narrow bandwidth filter for detecting from the synchronously detected random data the repetitive data segment syncs as described in U.S. Pat. No 5,416,524, assigned to Zenith, Electronic Corp. A control voltage error signal from the data segment sync detector 60 locks the symbol clock to the incoming data clock frequency.
As television broadcasters convert to digital transmission, precision test equipment to monitor transmitter performance will be needed. In analog broadcast, attention to the quality of the transmitted signal is driven primarily by the pride of the operator and somewhat by competitive pressures. The result of a poor quality transmitted signal is a degraded but still viewable picture at the receiver. In digital broadcasting, any artifacts inserted into the signal by the transmitter are interpreted by the receiver as noise. It appears that when the transmitter's signal to noise ratio falls below 27 dB, there will be a noticeable loss of coverage area as the transmitter's errors combine with normal environmental noise to drive receivers below the threshold of reception in fringe areas located well away from the transmitter. Threshold is an effect where the forward error correction in the digital television receiver successfully corrects errors in the digital television signal down to a point where an increase in the noise level or decrease of the signal level will swamp the forward error correction circuitry with the result of total picture loss. This is called the "cliff effect". Decreasing the signal to noise ratio of an analog NTSC signal results in increasing poor picture quality whereas decreasing the signal to noise ratio results in no loss of picture quality until the cliff effect kicks in and the picture is lost. Therefore, in digital broadcasting, loss of coverage is a direct economic consequence of poor transmitter signal quality. A strong emphasis on measuring and maintaining the digital television transmitter at optimum levels is to be expected.
There are a number of drawbacks to the above described VSB digital television receiver that makes it unsuitable for making precise digital television transmitter measurements. Extensive filtering of the digital signal in the receiver masks transmitter generated phase and magnitude variations and phase noise that may be present resulting in inaccurate transmitter measurements. Further, mixing the digital signal down to baseband in the receiver's final stages introduces spurious responses, such as intermodulation products and DC offset, in the output of the A/D conversion. The DC offset may be caused by the pilot signal in the digital signal or by the receiver itself with no way of distinguishing the difference between the two. Additionally, it is more difficult to hold the extreme flatness and envelope delay specifications when converting to baseboard band. Also, if only the I-channel is converted to baseband, there is a modest amount of spectral folding and aliasing that increases measurement error.
A Hewlett-Packard HP 89440A Vector Signal Analyzer has been used for making measurements on 8-VSB signals. The HP 89440A includes a superheterodyne receiver having a first LO and mixer for up-converting the incoming signal to a first IF frequency. Second and third LOs and mixers respectively generate second and third IF frequencies of 40 MHz and 10 MHz. The 10 MHz IF is digitized by an analog-to-digital converter with the digitized data being down converted to baseband real and imaginary data. The real and imaginary data values are passed to a digital signal processor for FFT conversion and additional signal processing. A limitation of the superheterodyne type receiver is the need for bandwidth limiting filters between the IF stages to prevent the undesired mixer signal outputs from entering subsequent IF stages. Such filtering can mask artifacts in the transmitter signal resulting in inaccurate measurements of the operating condition.
What is needed is an instrumentation receiver for digitally modulated radio frequency (RF) signals, such as VSB digital television signals, that accurately converts the incoming signal to an intermediate frequency (IF) signal without masking artifacts in the transmission signal. The instrumentation receiver should reduce the number of components, filters and design complexity associated with superheterodyne type receivers. The instrumentation receiver should further have a flexible design for receiving many different types of digitally modulated RF signals. In addition, the instrumentation receiver should facilitate the use of software based demodulation processes so that few hardware changes are required to adapt the design for different modulation standards.